feat(extension): add Firefox WebExtension support

- Add a neutral WebExtension API adapter that uses Firefox browser.* or Chromium chrome.* without mutating globals.
- Switch extension runtime code to the adapter and add Firefox-specific typings for tabs, windows, tab groups, storage, scripting, and native messaging ports.
- Fix Firefox temporary add-on instructions to load the packaged manifest with background.scripts instead of the Chromium service worker manifest.
- Detect Firefox in clients.list via runtime.getBrowserInfo and keep Chromium user-agent fallback support.
- Make navigate.open wait briefly for Firefox to replace initial about:blank with the requested URL.
- Add JS coverage for API selection, clients.list browser detection, and Firefox navigate.open URL polling.
- Bump package and extension version to 0.15.2.

tests/test_cli.py

@@ -140,6 +140,10 @@ def test_install_writes_firefox_allowed_extensions(tmp_path):
     }
   ]
   assert "about:debugging#/runtime/this-firefox" in result.output
+  assert "npm run package:extension:firefox" in result.output
+  assert "dist/extension-package-firefo" in result.output
+  assert "x/manifest.json" in result.output
+  assert "Do not select extension/manifest.json" in result.output
   assert "Firefox extension ID" in result.output
 
 def test_install_windows_registers_native_host(tmp_path):